Question 561203
Let {{{ x }}} = number of tests needed with score of
94% to average over 90%
{{{ ( 4*84 + 94x ) / ( 4 + x ) = 90 }}}
{{{ ( 336 + 94x ) / ( 4 + x ) = 90 }}}
{{{ 336 + 94x = 90*( 4 + x ) }}}
{{{ 336 + 94x = 360 + 90x }}}
{{{ 4x = 360 - 336 }}}
{{{ 4x = 24 }}}
{{{ x = 6 }}}
7 is the  number of tests needed with score of
94% to average over 90% 
( 6 test gives exactly 90% average )
check answer:
{{{ ( 4*84 + 94x ) / ( 4 + x ) = 90 }}}
{{{ ( 4*84 + 94*6 ) / ( 4 + 6 ) = 90 }}}
{{{ 336 + 564 = 90*10 }}}
{{{ 900 = 900 }}}
OK